Electroporation and electrofusion are related phenomena with a variety of uses in manipulation of prokaryotic and eukaryotic cells. Electroporation is the destabalization of cell membranes by application of a brief electric potential (pulse) across the cell membrane. Properly administered, the destabalization results in a temporary pore in the membrane through which macromolecules can pass while the pore exists. Electrofusion is the fusion of two or more cells by application of a brief electric potential across a cell membrane. The physical and biological parameters of electrofusion are similar to those of electroporation.
The potential applied to cell membranes is applied using instruments delivering various pulse shapes. The two most common pulse shapes are exponential decay and rectangular wave. The exponential decay pulse is generated with capacitance discharge pulse generators. It is the least expensive pulse generator and gives the operator the least control over pulse parameters. The rectangular wave pulse generator is more expensive, gives more control over pulse parameters and generates a pulse that is less lethal to cells. With both pulse shapes, the energy needed to generate resealable pores in cells is related to cell size, shape, and composition.
With electrofusion, cells must be in contact at the time of membrane destabalization. This is accomplished by physical means such as centrifugation, biochemical means such as antibody bridging, or electrical means through dielectrophoresis. Dielectrophoresis is the creation of a dipole within a cell by application of a low voltage potential across a cell membrane in an uneven electrical field. The dipole can be created in Unipolar or AC fields. Since Unipolar fields tend to generate unacceptable heat, radio frequency AC is often used for dielectrophoresis.
The uses of electroporation and electrofusion are many. A partial list follows: (1) transient introduction of DNA or RNA into both eukaryotic and prokaryotic cells; (2) permanent transfection of DNA into both eukaryotic and prokaryotic cells; (3) permanent and temporary transfection of DNA into human and animal cells for gene therapy; (4) introduction of antibodies, other proteins, or drugs into cells; (5) production of antibody producing hybridomas; (6) pollen electrotransformation in plants; (7) electroinsertion; (8) manipulation of animal embryos; (9) electrofusion of adherent cells; (10) production of plant somatic hybrids; (11) DNA vaccination; and (12) cancer therapy.
One of the ways that electroporation or electrofusion works is to induce the formation of holes or pores in the cell membrane. There is some controversy about the exact nature of the cell pore induced by the application of an electrical pulse to a cell, but the practical effect is an induced cell permeability and a tendency to fuse with other similarly affected cells that are in close contact.
There is a Unipolar voltage threshold for the induction of pores in or for the fusion of cell membranes. Voltages below the threshold will not bring about substantial cell membrane disturbance. The threshold potential for many cells is approximately one volt across the cell membrane. The total voltage applied per centimeter between electrodes to achieve one volt potential across the cell membrane is therefore proportional to the diameter of a cell. Small cells such as bacteria, require high voltages while larger cells, such as many mammalian cells, require somewhat lower voltages. There are other cell specific variables such as the structure of the cellular cytoskeleton that affect the voltage required for that cell.
When using electrical pulses which are powerful enough to bring about electroporation or electrofusion of cells, the main problem is that the process is often lethal to an unacceptable percentage of the cells. The lethality rate may be 50% or higher. There are a number of reasons why such high lethality rates to cells are not desirable. When cells are treated for further use in ex vivo gene therapy, lethality to the cells will prevent an adequate number of cells from uptaking therapeutic genetic material. When in vivo gene therapy is employed in a patient, lethality to cells may not only result in less effective treatment, but may also result in injury to the patient.
A number of methods have been used to reduce cell killing in electroporation and electrofusion. The most commonly used method is to apply a rectangular shaped pulse to cells instead of an exponential decay pulse. This method reduces the total energy applied to the cell while applying enough voltage to overcome the threshold. While the rectangular shaped pulse is an improvement, there is still substantial cell killing during an effective application of electrical energy to the cells.
Rectangular wave pulsers currently marketed for electroporation and electrofusion have a number of adjustable parameters (voltage, pulse width, total number of pulses, and pulse frequency). These parameters, once set, are fixed for each pulse in each pulse session. For example if a voltage of 1,000 volts per centimeter, pulse width of 20 microseconds, pulse number equal to 10, and a pulse frequency of 1 Hz is chosen, then each of the 10 pulses will be fixed at 1000 volts per centimeter and 20 microseconds for the pulse session.
However, even when using rectangular wave pulsers that employ fixed pulse parameters, an undesirably high lethality rate of the cells may still occur. In this respect, it would be desirable if wave pulses could be controlled in such a way that the lethality rate of cells would be significantly reduced.
In an article by Sukharev et al entitled "Electroporation and electrophoretic DNA transfer into cells" in Biophys. J., Volume 63, November 1992, pages 1320-1327, there is a disclosure that two generators are employed to generate pulses. A time delay switch controls a first pulse generator to generate a first pulse to be imposed on biological Cos-1 cells. The first pulse has an amplitude sufficient to induce pore formation in the cells. The time delay switch causes a time delay and then controls a second pulse generator to generate a second pulse which is imposed on the cells. The second pulse is insufficient to sustain the induced pores formed from the first pulse. However, the second pulse is sufficient to bring about electrophoresis of DNA material into the previously pulsed cells. Several key points are noted with respect to the disclosures in the Sukharev et al article. First, the induced pores that are formed in the cells as a result of the first pulse begin to contract after the first pulse is over without any additional pulse being imposed on the cells sufficient to sustain the induced pores. Second, the Sukharev et al article does not address the issue of cell viability after the induced-pore-forming pulse. Third, there are only two pulses provided with Sukharev et al. Therefore, the time period that the DNA material can enter the cells is constrained by the effects of only two brief pulses. In this respect, it would be desirable if a pulse protocol were provided that sustains induced pores formed in electroporation. Moreover, it would be desirable if a pulse protocol were provided which is directed towards improving cell viability in cells undergoing electroporation. Furthermore, it would be desirable if a pulse protocol were provided which provides three or more pulses to allow more time for materials to enter cells undergoing electroporation.
In an article by Ohno-Shosaku et al entitled "Somatic Hybridization between Human and Mouse Lymphoblast Cells Produced by an Electric Pulse-Induced Fusion Technique" in Cell Structure and Function, Vol. 9, (1984), pages 193-196, there is a disclosure of the use of an alternating electric field of 0.8 kV/cm at 100 kHz to fuse biological cells together. It is noted that the alternating current provides a series of electrical pulses all of which have the same duration, the same magnitude, and the same interval between pulses.
In an article by Okamoto et al entitled "Optimization of Electroporation for Transfection of Human Fibroblast Cell Lines with Origin-Defective SV40 DNA: Development of Human Transformed Fibroblast Cell Lines with Mucopolysaccharidoses (I-VII)" in Cell Structure and Function, Vol.17, (1992), pages 123-128, there is a disclosure that a variety of electric parameters were tested to obtain optimum electroporation. The electric parameters included voltage, pulse-duration, the number of pulses, and pulse shape. It is noted that for any particular set of electric parameters that were selected, all of pulses with the selected parameters had the same duration, the same magnitude, and the same interval between pulses.
In an article by Orias et al entitled "Replacement of the macronuclear ribosomal RNA genes of a mutant Tetrahymena using electroporation" in Gene, Vol. 70, (1988), pages 295-301, there is a disclosure that two different electroporation devices were employed. It is noted that each electroporation device provided a series of electrical pulses (pulse train) for each electroporation run. For any particular electroporation run, all of the pulses in the pulse train had the same duration, the same magnitude, and the same interval between pulses.
In an article by Miller et al entitled "High-voltage electroporation of bacteria: Genetic transformation of Campylobacter jejuni with plasmid DNA" in Proc. Natl. Acad. Sci USA, Vol. 85, February 1988, pages 856-860, there is a disclosure that a variety of electric pulse parameters were tested to obtain optimum electroporation. The electric pulse parameters included field strength and time constant. It is noted that for any particular set of pulse parameters that were selected, all of pulses with the selected parameters had the same field strength and the same time constant.
In an article by Ogura et al entitled "Birth of normal young after electrofusion of mouse oocytes with round spermatids" in Proc. Natl. Acad. Sci USA, Vol. 91, August 1994, pages 7460-7462, there is a disclosure that oocytes were electroactivated by exposures to AC (2MHz, 20-50 V/cm for 10 seconds) and Rectangular (1500 V/cm for 80 microsec.) pulses in Dulbecco's PBS medium. Following electroactivation, the cells were removed from the Dulbecco's PBS medium and placed in a Hepes/Whitten medium and injected with single spermatids. The oocyte-spermatid pairs were placed in fusion medium and exposed to, in sequence, an AC pulse (2MHz, 100V/cm, for 15-30 seconds), a fusion Rectangular pulse (3750-4000 V/cm for 10 microsec.), and a subsequent AC pulse (2 MHz, 100V/cm for 15-30 seconds). The time interval between application of the oocyte activation pulse and the oocyte-spermatid fusion pulse was 15-40 minutes. Several points are noted with respect to the disclosures in the Ogura et al article. First, electroporation is not conducted; instead, electrofusion is conducted. Moreover, entry of the spermatid into the oocyte is by injection, not electroporation. Second, only two Rectangular pulses are employed. Neither of the AC pulses has a sufficient voltage level to provide an electroporation threshold. The sequence of two Rectangular pulses are not disclosed as having induced pore formation. Pore formation is not utilized in this method of cell fusion. No provision is made to sustain pores formed or to maintain viability of cells treated.
In an article by Andreason et al entitled "Optimization of electroporation for transfection of mammalian cell lines" in Anal. Biochem., Vol.180, No.2, pages 269-275, Aug. 1,1989, there is a disclosure that transfection by electroporation using square wave pulses, as opposed to exponentially decaying pulses, was found to be significantly increased by repetitive pulses. Different pulse amplitudes were tried in different experimental runs to determine the effects of different electric field strengths. For a given experimental run, each pulse has the same voltage and same duration as each other pulse.
In an article by Vanbever et al entitled "Transdermal Delivery of Metoprolol by Electroporation" in Pharmaceutical Research, Vol. 11, No. 11, pages 1657-1662, (1994), there is a disclosure that electroporation can be used to deliver drugs across skin tissues. The article discloses a series of electroporation experiments for the purpose of determining optimum electroporation conditions. An electroporation apparatus was employed that coud be programmed to produce three types of pulses based on exponentially decaying capacitive discharge: a single HV pulse (ranging from 3500V to 100V; a single LV pulse (ranging from 450V to 20V); and a twin pulse consisting of a first HV pulse, and interpulse delay (1 second), and a second LV pulse. If more than one pulse were applied, they were separated by 1 minute. It is noted that none of the pulses applied are rectangular in shape. In actual experiments run, using a twin pulse, the second LV pulse had a pulse amplitude of 100 volts (see FIGS. 1 and 2 on page 1659). As a result of comparisons made between the results of actual experiments conducted, it was concluded in the second column on page 1659 that "single pulse was as efficient as the twin pulse to promote metoprolol permeation, indicating that twin pulse application was not necessary". Moreover, a further conclusion beginning in the same column of the same page and extending to the first column of page 1660 states "long pulse (621.+-.39 ms) at a low voltage was much more efficient than a high voltage pulse with a short pulse time (3.1.+-.0.1 ms) to promote metoprolol permeation". Beginning in the first paragraph of the first column on page 1660, the authors state "The short high voltage pulses used in this study hardly had any effect, while pulses of hundreds of volts and a few ms time constants were reported to have dramatic effect on transdermal permeation". Clearly, Vanbever et al teach away from using a pulse train having pulses of different amplitudes. Moreover, nothing in the Vanbever et al article relates to the issue of cell viability for cells undergoing electroporation.
In an abstract of an article by Bahnson et al entitled "Addition of serum to electroporated cells enhances survival and transfection efficiency" in Biochem. Biophys. Res. Commun., Vol. 171, No. 2, pages 752-757, Sep. 14, 1990, there is a disclosure that serum rapidly reseals the membranes of electroporated cells and that timely addition of serum following electroporation can improve cell survival and transfection efficiency. Rather than require the use of serum, it would be desirable if an electrical way were provided to improve cell survival and transfection efficiency.
In an abstract of an article by Knutson et al entitled "Electroporation: parameters affecting transfer of DNA into mammalian cells" in Anal. Biochem., Vol. 164, No. 1, pages 44-52, July 1987, there is a disclosure of an instrument which permits the high-voltage discharge waveform to be varied with respect to rise time, peak voltage, and fall time. Tests were done in which the mammalian cells were exposed to multiple voltage discharges, but the multiple exposures did not improve DNA transfer. It is noted that with the use of multiple pulses, each pulse has the same voltage and same duration as each other pulse.
In an abstract of an article by Kubiniec et al entitled "Effects of pulse length and pulse strength on transfection by electroporation" in Biotechniques, Vol. 8, No. 1, pages 16-20, January 1990, there is a disclosure that the relative importance of pulse field strength E and pulse length tau 1/2 (half decay time of an exponential decay pulse) were investigated for HeLa or HUT-78 cells. In the abstract, there is no disclosure of using a plurality of pulses for carrying out the electroporation.
Throughout the years a number of innovations have been developed in the fields of electroporation, electrofusion, dielectrophoresis, and the U.S. patents discussed below are representative of some of those innovations.
U.S. Pat. No. 4,441,972 discloses a device for electrofusion, cell sorting, and dielectrophoresis which includes specially designed electrodes which provide a non-uniform electrical field. The non-uniform electric fields are used for sorting cells. More specifically, at least one of the electrodes has a surface which includes a plurality of substantially concentric grooves. Because preparation of such concentric-groove-containing electrodes may be expensive and time consuming, it would be desirable if an electrofusion device could be provided that provides variations in electric fields applied to living cells without the use of electrodes having a plurality of concentric grooves.
The device in U.S. Pat. No. 4,441,972 can be used for cell sorting by dielectrophoresis. For cell sorting, the frequency and intensity of an AC voltage that is applied to the electrodes may be varied so that the cells which are desired for collection will arrive at a predetermined radial distance from an opening port and then later be collected and withdrawn through an exit port when the field is relaxed. Rectangular electrical pulses are not used for cell sorting.
The device in U.S. Pat. No. 4,441,972 can also be used for electrofusion. With this manner of use, a low AC voltage is applied to the electrodes in order to allow the cells to contiguously align between the electrodes. Typically a mild AC field of about 10 volts rms at about 250 Khz may be utilized. Then, a single brief unipolar pulse of about 10 to about 250 volts for about 50 microseconds may be applied to cause fusion of the aligned cells. The patent discloses that the frequency, voltage and duration of impulse may be adjusted depending on the type and size of cells to be sorted or fused or upon the type of carrier stream used. In the patent, there is disclosure that various devices, including computers, can be used to control input frequency and voltage to the electrode. However, with particular attention being paid to electrofusion, in U.S. Pat. No. 4,441,972, there is no disclosure of using more than a single pulse for carrying out the electrofusion. It is noted that U.S. Pat. No. 4,476,004 is closely related to U.S. Pat. No. 4,441,972 and has a similar disclosure.
U.S. Pat. No. 4,695,547 discloses a probe for electrofusion, electroporation, and the like. A suitable source of high voltage pulses is disclosed as being capable of providing output voltage pulses in the range of 25-475 Volts at currents up to 1 amp and durations of 1-990 ms. There is no disclosure of using a plurality of Rectangular pulses for carrying out electrofusion or electroporation.
U.S. Pat. No. 4,750,100 discloses a high-voltage and high-amperage switching device capable of delivering an exponential decay pulse or a rectangular wave pulse for electroporation. There is no disclosure of using a plurality of Rectangular pulses for carrying out electroporation or transfection.
U.S. Pat. No. 4,832,814 discloses an electrofusion cell that is used for conducting electrofusion of living cells. An electrical power source provides a series of three pulses, each of 12 microsecond and of 68 volts with a 1 second separation between pulses. It is noted that each pulse has the same voltage and same duration as each other pulse.
U.S. Pat. No. 4,882,281 discloses a probe for electrofusion, electroporation, and the like. Just as disclosed in U.S. Pat. No. 4,695,547 described above, a suitable source of high voltage pulses is disclosed as being capable of providing output voltage pulses in the range of 25-475 Volts at currents up to 1 amp and durations of 1-990 ms. There is no disclosure of using a plurality of pulses for carrying out electrofusion or electroporation.
U.S. Pat. No. 4,910,140 discloses a method for the electroporation of prokaryotic cells by applying high intensity electric fields of short duration. This patent discloses that the pulse will normally consist of a single pulse comprising the entire desired period. Alternatively, the pulse may consist of a plurality of shorter pulses having a cumulative time period coming with desired 2 to 20 msec range. Such a series of pulses must be spaced sufficiently close to one another so that the combined effect results in permeabilization of the cell wall. Typically, such pulses are spaced apart by 5 msec or less, more preferably being spaced apart by 2 msec or less.
U.S. Pat. No. 4,955,378 discloses electrodes for delivering pulses to animal or human anatomical sites for carrying out in vivo electrofusion. It is disclosed that, generally, electrofusion is preferably accomplished under constant voltage conditions by applying to the electrode 3 square waves of 20 volts amplitude and of 20 microsecond duration at a pulse rate of 1 pulse per second. It is noted that each pulse has the same voltage and same duration as each other pulse.
U.S. Pat. No. 5,007,995 discloses a device for electrofusion of living cells. Instead of using unipolar pulses, AC pulses were employed. A series of studies were conducted among the variables of AC frequency, AC voltage applied, and the time duration of the AC pulse. In each study, two of the three variables were held constant, and one variable was varied by setting the variable at different incremental settings. There is no disclosure of using a plurality of Rectangular pulses for carrying out electrofusion.
U.S. Pat. No. 5,019,034 discloses a method for electroporating tissue for the purpose of transporting large molecules into the tissue. Frog skin is used as an example. In carrying out the electroporation, the shape, duration, and frequency of the pulse are selected. The peak voltage is also placed at an initial setting. The pulse is gradually cycled to higher voltages until electroporation occurs. At that point, the pulse shape, duration, frequency, and voltage are maintained until a desired amount of molecular transfer has occurred.
U.S. Pat. No. 5,124,259 discloses a method of electroporation using a specifically defined buffer in which the chloride ion is eliminated. There is a disclosure that, in carrying out the electroporation, the electric field may be 100-1000 V/cm and the time for applying the voltage may be 0.1-50 msec. There is no disclosure of using a plurality of pulses for carrying out the electroporation.
U.S. Pat. No. 5,128,257 discloses several chambers and electrodes used for electroporation. Power supplies provide a voltage range of 200 to 2000 volts. The pulse width is in a range from 0.1 to 100 milliseconds, preferably 1 to 10 milliseconds. There is no disclosure of using a plurality of pulses for carrying out the electroporation.
U.S. Pat. No. 5,134,070 discloses a specially coated electrode on which cells are cultivated. The cells on the electrode are subjected to electroporation. In carrying out the electroporation, a device for measuring electrical field intensity is appropriately interfaced to a micro-processor so that an "intelligent" electroporation device is provided which is capable of applying an ever increasing electrical potential until the cells have porated and which is capable of sensing at what field intensity the cells have porated. Since the device measures the conditions required to induce poration, and detects when poration occurs, substantial reductions in current mediated cell death will be realized since only enough energy to induce poration is introduced into the system. However, it is noted that there is no disclosure of using a plurality of pulses for carrying out electroporation. In addition, the electroporation device is capable of recording information concerning the poration potential required for various cell lines and the effects of various media compositions on the types and sizes of porations that may occur. It is noted, however, that provisions are not made to sustain pore formation that has been induced.
U.S. Pat. No. 5,137,817 discloses an apparatus and method for electroporation using specially designed electrodes for conducting electroporation in vivo. In carrying out the electroporation, a single Unipolar voltage pulse is applied to the host cells. There is no disclosure of using a plurality of pulses for carrying out electroporation.
Each of U.S. Pat. Nos. 5,173,158 and 5,283,194 discloses an apparatus and methods for electroporation and electrofusion in which an electrode is employed that selectively admits cells of a certain size and excludes others. A single pulse generates an electric field which causes electroporation. There is no disclosure of using a plurality of pulses for carrying out either electroporation or electrofusion.
U.S. Pat. No. 5,186,800 discloses, as does U.S. Pat. No. 4,910,140 discussed above, a method for the electroporation of prokaryotic cells by applying high intensity electric fields of short duration. U.S. Pat. No. 5,186,800 also discloses that an applied pulse will normally consist of a single pulse comprising the entire desired period. Alternatively, the pulse may consist of a plurality of shorter pulses having a cumulative time period coming with desired 2 to 20 msec range. Such a series of pulses must be spaced sufficiently close to one another so that the combined effect results in permeabilization of the cell wall. Typically, such pulses are spaced apart by 5 microsec. or less, more preferably being spaced apart by 2 microsec. or less. A series of experiments were conducted to ascertain method parameters which provided maximum cell transformation. In each of the experiments, a single electrical pulse was used to bring about electroporation. Experimental parameters included a number of parameters of the electrical pulse, concentration of the host cells, concentration of the transforming material, and post-shock incubation period. It was observed that the viability and transformability of the cells undergoing electroporation were very sensitive to the initial electric field strength of the pulses. A conclusion reached was that cell survival declines steadily with increasing field strength; and in each of the experiments conducted, the maximum transformation efficiency is reached when 30 to 40% of the cells survive the pulse. There is no disclosure of using a plurality of pulses for carrying out electroporation. In view of the above, it would be desirable if a method of electroporation were provided in which the maximum transformation efficiency were achieved when greater than 40% of cells survive the pulse effecting electroporation.
U.S. Pat. No. 5,211,660 discloses a method for performing an in vivo electrofusion. Details relating to electrical parameters of a unipolar electric field that is utilized are not disclosed.
U.S. Pat. No. 5,232,856 discloses an electroporation device which employs specially designed electrodes. A number of electroporation experiments were conducted using a number of different host cells and different transforming material. In each experiment, only a single Unipolar pulse was applied to the host cells. There is no disclosure of using a plurality of pulses for carrying out electroporation.
U.S. Pat. No. 5,273,525 discloses a syringe based electroporation electrode for drug and gene delivery. In using the electroporation electrode, a conventional power supply is employed which provides from one to one hundred consecutive pulses having a constant pulse amplitude, a constant pulse width, and a constant pulse interval.
Each of U.S. Pat. Nos. 5,304,120 and 5,318,514 discloses an electrode for in vivo electroporation of living cells in a person. In applying electrical energy for bringing about electroporation, a power supply preferably applies electric fields repeatedly, and the amplitude and duration of the electric fields make the walls of the living cells sufficiently permeable to permit drugs or genes to enter the living cells without killing them. The power supply includes a unipolar oscillating pulse train and a bipolar oscillating pulse train. It is noted that, for a chosen pulse train, each pulse rises to the same voltage and has the same duration as each other pulse in a pulse train.
Having discussed a number of theoretical considerations and a number of prior art disclosures, attention is now returned to a further discussion of certain theoretical concepts relating to induction of pore formation in biological cells. It is understood, however, that none of the theoretical concepts discussed herein are intended to limit the scope of the invention. Instead, the scope of the invention is limited only by the claims appended hereto and equivalents thereof.
It has been discovered by the inventors of the present invention that changing pulse parameters during a pulse session reduces damage to cells while maintaining or improving electroporation and electrofusion efficiency. The reduced cell damage can be related to reduced energy applied to the cell. More specifically, two parameters determining total energy applied per pulse are pulse amplitude and pulse width. Variation of pulse width would have different effects than variation of pulse amplitude. Reduction of pulse width following application of a wider pulse would permit application of an above threshold voltage while reducing the total energy in a series of pulses.
Furthermore, for theoretical reasons described below, maintenance of pores already formed in a cell should require less energy than the energy required to initiate a new pore. Variation of pulse width while maintaining an above threshold voltage would be particularly useful in those instances where very small pores are initiated by a wider pulse. Narrower pulses could assist pore expansion in a controlled manner. The ideal condition for any particular type of cell would be to find a set of electrical pulse parameters that would cause pore expansion to a size large enough to permit a foreign molecule (such as a small organic molecule or DNA) to enter the cell without expanding the pore size to one beyond recovery. The pulse parameters to accomplish this goal would have to be experimentally determined for each cell type.
Variation of pulse amplitude would permit application of a below threshold maintanance pulse. Once a pulse of sufficient energy with an above threshold voltage is applied to a cell, a transient decrease in electrical resistance across the cell membrane occurs. Because of the decreased electrical resistance of the cell membrane, pulse voltages below threshold should be sufficient to maintain a cell pore induced by an above threshold pulse.
Thus, while the foregoing body of prior art indicates it to be well known to use electrical pulses to induce electroporation, the prior art described above does not teach or suggest a method of treating materials with pulsed electrical fields which has the following combination of desirable features: (1) provides a process for application of a series of electrical pulses to living cells wherein the electrical pulses produce reduced cell lethality; (2) provides an operator of electrical pulse equipment a process for maximum operator control of an applied pulse series; (3) provides a process for changing pulse width during a series of electrical pulses; (4) provides a process for changing pulse voltage and therefore electric field during a series of electrical pulses; (5) provides a machine for control of the process; (6) provides a pulse protocol that sustains induced pores formed in electroporation; (7) provides a pulse protocol which provides three or more pulses to allow more time for materials to enter cells undergoing electroporation; (8) provides an electrical way to improve cell survival and transfection efficiency; and (9) provides a method of electroporation in which maximum transformation efficiency is achieved when greater than 40% of cells survive the pulse effecting electroporation. The foregoing desired characteristics are provided by the unique method of treating materials with pulsed electrical fields of the present invention as will be made apparent from the following description thereof. Other advantages of the present invention over the prior art also will be rendered evident.